The present invention is in the field of glass-plastic composites, and particularly relates to glass-plastic laminated blanks for optical and ophthalmic lenses which resist shaling fracture during lamination and finishing processes.
It has been proposed to provide laminated articles comprising glass and plastic layers which could combine the desirable properties of both plastics and glasses, e.g., the lightweight and toughness of plastics and the scratch resistance or light-responsive characteristics of glasses. For example, German Auslegeschrift No. 1,284,588 by Gliemeroth describes laminated articles comprising plastic and photochromic glass layers which could be used to provide glass-plastic laminated lenses exhibiting photochromic properties.
A particularly desirable glass-plastic laminate for optical and ophthalmic applications is a laminate comprising a thin sheet glass core element composed, for example, of photochromic glass, which is positioned between two relatively thick plastic surface layers which are bonded to the front and back surfaces of the glass core and substantially cover the front and back surfaces thereof. Such a laminate combines the very desirable properties of photochromic behavior and light weight. These laminates may be produced by the high-temperature lamination of the glass and plastic components using preformed plastic and glass sheet or by directly casting plastic resins against the glass core and curing to form the covering surface layers. The copending, commonly assigned application of S. T. Gulati et al., Ser. No. 018,107, filed Mar. 7, 1979, describes direct-cast laminated glass-plastic lenses.
One problem which has been encountered in the manufacture of laminated glass-plastic lens blanks by these processes has been the shaling fracture of the glass core which can occur as the laminated lens blank is cooled to room temperature after processing, or as it is subsequently handled and finished. The opposing plastic front and back surface layer elements, to which the glass core is strongly bonded, exhibit substantial shrinkage with respect to the glass as the laminate is cooled to room temperature following processing. This shrinkage exerts considerable tensile stress on the core, referred to as bond stress because it is in a direction normal to the glass-plastic bonding plane at the outer circumferential edge of the glass core. In the presence of this stress, midplane or shaling fracture of the glass core can be initiated by any surface defect present on the core edge. The end result is separation of the lens blank into the two plastic surface layers, each retaining a portion of the glass core layer bonded thereto.
It is a principal object of the present invention to provide a novel configuration for a laminated glass-plastic lens blank comprising a thin glass core, which lens blank exhibits improved resistance to shaling fracture of the glass core during manufacture and subsequent handling of the lens blank.
Other objects and advantages of the invention will become apparent from the following description thereof.